'Trajectory analysis for the OTAG domain...' is one of 10 general tasks
identified on the OTAG Air Quality Analysis (AQA) 'wish list'. While no
extramural funding costs were identified to support this task, the States
of VT and TX had indicated some ability to contribute in-kind services.
During a 1/24/96 AQA Workgroup conference call, several participants expressed
capability/interest in assisting with this task. Subsequently, several
other individuals have expressed interest in assisting with OTAG 'Trajectory
Analysis' Tasks, and it has been suggested that we form an 'Ad Hoc Air
Trajectory group' (AHAT) within the OTAG Air Quality Analysis Workgroup.
This memo is intended to initiate discussions in the following areas:

What are the most effective means of communication and collaboration
among participants of the OTAG AHAT group? (Yet another series of bi-weekly)
conference calls, (another 100 pages/week of "urgent") faxes,
and (another series of expensive, time consuming) face to face meetings
- are not conducive to "doing work" (although occasional conference
calls, faxes and meetings are desirable). The interactive structure, function
and content of the OTAG Air Quality Analysis Internet Server (http://capita.wustl.edu/OTAG)
provide an effective, efficient vehicle for exchange of technical ideas,
data, results and reports. Group or individual Internet E-mail, and Internet
FTP file transfer could also facilitate exchange of "short" communications
and "large" data files. For those who lack Internet connections,
you almost certainly have a local Internet access provider who offers a
modem-based connection for all of the above at a very modest cost (less
than $1/hour of use). Commercial services - such as America On Line and
CompuServe - offer full Internet access at a nominal cost of about $10/
month. I think the advantages of organized electronic communications outweigh
the limitations, and will gladly provide technical assistance to any AHAT
participant who lacks connections - via telephone at (802) 241-3840.

What's a Trajectory?

Trajectory analysis is not a precisely-defined term, and may need some
clarification for OTAG purposes. In the simplest sense, a trajectory is
an estimation of the path of air movement over space and time. While meteorologists
often calculate trajectories "by hand" from atmospheric meteorological
data, calculations are commonly facilitated by several currently available
computerized trajectory models, such as ARL-ATAD (Heffter, 1981), HY-SPLIT
(Draxler, 1992) and CAPITA Monte Carlo (Schichtel, 1995). Trajectory models
require input of gridded, 4 dimensional meteorological data - which itself
must be derived from a meteorological model, such as NGM (Rolph, 1992),
RAMS(Pielke, 1992), etc. Trajectories are typically run in a backward
mode (path of air movement arriving at a receptor location) or forward
mode (path of air movement leaving from a source location). They can also
be calculated with alternative assumptions of critical atmospheric physics,
for example, at fixed (or mixed) elevations, pressures or isentropes in
the atmosphere.

Foreword or backward trajectory model output is most commonly displayed
as a "simple" 2 dimensional (horizontal) path of air movement
on a map view. However, the model calculations and output include substantial
additional information - including vertical motion, and various meteorological
characteristics (pressure, humidity, clouds, precipitation, temperature,
solar radiation, mixing heights, etc.) along the trajectory path). A more
comprehensive term to incorporate this important supplemental information
is "Airmass History". The gridded photochemical models are, in
effect, calculating forward airmass histories for thousands of individual
release locations and start times (along with the complex simultaneous
influences of fluctuating emissions and atmospheric chemistry).

It should be recognized that any model-derived "Airmass History"
is, by definition, a "Revisionist History". Results are dependent
on: which trajectory model, which met driver(s), which physical assumptions
on horizontal and vertical movement and/or mixing, which starting elevations,
which surface influences on flows, etc. Likewise, pollution at a given
receptor location has invariably resulted from emissions from a large number
of source locations; while emissions from an individual source location
ultimately influence a wide range of receptor sites. Where did today's
ozone come from? From the models, as from the atmosphere, we should anticipate
more than one answer.

What's the Use?

What are the potential uses of trajectory analysis (or airmass histories)
for OTAG data analysis?

For OTAG Model Episodes:

Illustration/visualization of meso-scale and synoptic-scale atmospheric
flow during model episodes

If there are no funds to support additional trajectory work, it seems
critical to couple the questions of "what to do" and "who
will (volunteer to) do it". At the same time, if good ideas are developed
which require supplemental funding support, the AHAT group should seek
to identify those potential tasks and funding needs (sometimes the system
works in mysterious ways -and good ideas can sometimes attract sufficient
funding support)

Specific Tasks:

Residence Time Analysis (and other ensemble / trajectory climatology
techniques) - VT DEC will participate, and is eager to collaborate with
other interested participants

What else?

Who else?

What's up in VT?

VT DEC has initiated some exploratory analysis using the NOAA HY-SPLIT
model and 1989-95 NGM met data. A 2/29/95 draft summary report is available
via OTAG Web server at (address), and is also available in WordPerfect
6.1 format via anonymous ftp at (address) or as hard copy (B&W-only)
from VT DEC.

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